Glycoconjugates may broadly be classified into three groups, i.e., glycolipid, glycoprotein and proteoglycan. The glycolipid may further be classified into glycoglycerolipid and glycosphingolipid. The latter has attracted widespread attention in recent years and is in active investigation. Glycosphingolipids containing sialic acids are specifically called gangliosides which are known to constitute diverse molecular species depending upon the structure of the saccharide moiety and the lipid moiety. Such molecular diversity of gangliosides is thought to be involved in the modulation of cell growth, differentiation and immunological function; marker molecules for oncogenesis; and receptors for compounds acting on cells, contributing to possible biological roles of gangliosides, which still remains to be determined.
One of the methods for elucidating the function of glycosphingolipids on a molecular level is addition of an exogenous glycosphingolipid, which is simple and convenient to run. It is reported that addition of glycolipids to a cell culture system results in incorporation of the ceramide moiety into the cell membrane [G. Schwarzman et al., Biochemistry, 22, 5041 (1983)]. On the other hand, it is also reported that exogenous ganglioside GM.sub.3 added to HL-60 cell culture system is quantitatively incorporated into the cells but is metabolized rapidly to a considerable degree [N. Nakamura et al., Biochem. Biophys. Res. Commun. 161, 782 (1989)]. This means that, with gangliosides, the exogenous molecules are more readily hydrolyzable by sialidase than the endogenous molecules.
In addition, the exogenous molecules are removed by enzymatic treatment in some cases. Therefore, many problems remain to be solved with respect to binding to the membrane, incorporation into cells, metabolism and functional expression of the exogenous molecules.
The hemagglutination reaction with influenza virus found by Hirst [G. K. Hirst, Science, vol. 94, 22 (1944)] suggested the presence of receptors for influenza virus on the cell membrane and the presence of hemagglutinin which recognizes and binds to the receptor, as well as the presence of receptor-destroying enzymes. Later, the receptor-destroying enzymes were found to be sialidases. The hemagglutinin and the sialidase are essential molecular species for establishment of infection and extracellular release of viruses such as adsorption to the receptor, destruction of the receptor, intracellular invasion associated with membrane fusion and liberation of the matured virus from the cell.
Various gangliosides have been used to investigate specific sialosyloligosaccharide structures in receptors recognized by the viruses bearing antigenically different hemagglutinin subtypes and it has been found that the structures are recognized in common despite difference in antigenicity [Y. Suzuki et al., J. Biol. Chem., 261, 17057 (1986)].
Recently, it was elucidated that S-glycoside analogs of gangliosides possessed an inhibitory activity for the influenza virus sialidase [Y. Suzuki et al., Glycoconjugate J. (1990) 7:349-356]. The S-glycoside analogs are said to be non-natural type molecular species since they would not naturally be found even if converted to an O-glycosides. When such S-glycoside analogs are used as a substrate for influenza virus sialidase, they were almost non-hydrolyzable, indicating inhibition of the sialidase. This shows that the S-glycoside analogs may act as competitive inhibitors of sialidase in the presence of GM.sub.3 -gangliosides. Furthermore, the compound of the following structure was investigated for inhibitory activity against sialidases of different influenza viruses, which was found to exhibit extensive inhibitory activities. ##STR4##
These have suggested that the S-glycoside analogs may be useful for broad-spectrum anti-influenza viral agents which are intended for the inhibition of budding of the virus from host cells. In recent years, an enzyme was discovered which may offer a clue to elucidating the true role of endogenous glycolipids. This enzyme hydrolyzes the glycosidic linkage between the oligosaccharide and ceramide, which is named endoglycoceramidase [M. Ito et al., J. Biol. Chem., 261, 14278 (1986)]. Studies have been reported on the function of the enzyme, the mode of substrate and the substrate specificity of the enzyme with no report on the true role of endogenous glycolipids [M. Ito et al., J. Biol. Chem., 264, 9510 (1989)]. An inhibitory activity of the S-glycoside analogs of gangliosides for endoglycoceramidase is expected as that of S-glycoside analogs for sialidases.
Thus the S-glycosides of gangliosides are shown to be inhibitors of the enzyme and they are much expected for their pontential capacities. In this respect, a novel class of S-glycosides is desired.
On one hand, gangliosides occur in living bodies only in a very small quantity and the specimen extracted and purified from living bodies are structurally diversified in the ceramide moiety. A single specimen is difficult to obtain in quantity by extraction. Thus it has been desired to supply a single specimen in large quantities for extensive biochemical studies of gangliosides.